Cluster impact at surfaces

Dipl.-Phys. Steffen Zimmermann

A) In most cases,
ion-impact induced sputtering of solids follows the well known law of linear-cascade theory. However, if the energy-density near the surface is very
high, the sputter yield gets much higher than can be expected from linear-cascade theory.

Using
molecular-dynamics simulation, we study the sputtering of a Au (111) surface due to impact of Aun (n = 1, 2, 4, 8, 13 and 43, 87, 134, 201) with energies in the range of 1 keV/atom up to 64
keV/atom. We measure the initial sputter yield Yinitial at the time when the maximum number of atoms are energized to above the cohesive energy of the material, the energy which is
deposited near the surface at that time, and the total sputter yield after the crystal is annealed (after 100ps). Than we correlate these early values
to the final sputter yield, Yfinal in order to
examine the influences of cluster size and impact velocity.

Additionally, for
the larger clusters, we determine that part of the yield which is due to reflected projectile atoms, not to the truly sputtered atoms.

(The figure shows
the Au4 (64keV/atom) impact at a crystal with 1.26
millions of atoms.)

B) We look for
the fragmentation behaviour of slow clusters (up to 20 eV). In this regard, we focus especially on the excitation of the cluster, its energy
distribution and the fragmentation pattern. We are interested in the difference between atomic clusters and molecular clusters and the influence of
their additional degrees of freedom on the stability of the cluster. (The figure shows the increase in the different energy forms related to the
increasing impact energy for a synthetic nitrogen-cluster with a weak intramolecular binding energy.)